A pneumatic tire as a typical example of the rubber articles, particularly a tire for construction vehicles is mounted onto a large-size dump truck or the like used in, for example, a large-scale civil engineering work site or an ore mining site, and subjected to severe operating conditions on rough road surfaces under a heavy load. This type of the tire is general to have a reinforcing structure that a carcass toroidally extending between a pair of bead cores is a skeleton and also a plurality of belt layers are arranged at an outside of the carcass in a radial direction.
As the above tire for the construction vehicle is particularly run on an irregular land having a violent unevenness under a heavy load, a tread of the tire is subject to a large deformation and hence a large compression force is repeatedly applied thereto. To this end, steel cords are used as a reinforcing member for the belt or the carcass in this type of the tire, wherein a strand construction such as 7×(3+9+15) construction or the like formed by twisting a plurality of strands, each being formed by twisting a plurality of filaments, is adopted for making a breaking load per a diameter of a cord large and providing a good resistance to fatigue.
As shown in FIG. 1, the cord having the 7×(3+9+15) construction comprises a core strand 10a formed by arranging two sheath layers 10b, 10c each made of plural filaments having the same diameter as in a core 10a around the core 10a made of three filaments, and six sheath strands 11 each having the same construction as mentioned above and twisted around the core strand. Moreover, numeral 12 is a wrapping filament.
On the other hand, in case of a cord having a single twisting construction or a layer construction formed by twisting a plurality of filaments, a sum of tenacities of the filaments as a constitutional element does not coincide with a tenacity of the cord but slightly decreases for each of the filaments inclined with respect to an axis of the cord (at a twisting angle) through the twisting.
This is true in the cord having the above strand construction. In case of the cord having the strand construction, particularly the cord of the strand construction made of high-tenacity filaments, there becomes a problem that the cord tenacity is decreased to an extent that can not be explained by the decrease resulted from the twisting angle as compared with the sum of the filament tenacities. That is, even when the cord of the strand construction is made of high-tenacity filaments, the improvement of the desired cord tenacity can not be attained.
In the cord of the strand construction to be targeted in the invention, as the core strand extends substantially straight in a central portion of the cord and is subjected to constriction from all sheath strands located around the core strand, stress concentration resulted from the constriction in the cord is apt to be caused in a portion contacting these strands with each other. Moreover, the conventional cord of the strand construction is a construction made of the filaments having a single diameter as shown in FIG. 1.
When such a cord of the strand construction is constituted with high-tenacity filaments, if tension is applied to the cord, it has been confirmed that the stress concentration resulted from the constriction of the core strand with each of the sheath strands becomes larger and a part of the filaments is broken prior to the cord breakage. In other words, when tension is applied to the cord, an outer-most sheath filament located in a contact portion between the strands is exposed to a state of being subjected to a compression load in a direction crossing with an axis of the filament while applying a tensile load. As a result, the breakage of the filament is caused due to shearing stress generated in the inside of the filament. In the cord of the strand construction, therefore, the breakage of the filament is particularly apt to be prematurely caused in outermost sheaths of the core strand and the sheath strand and a part of the filaments is precedently broken and hence the actual cord tenacity is decreased to an extent that can not be explained by the decrease resulted from the twisting angle as compared with the sum of tenacities of the filaments constituting the cord. This phenomenon is especially remarkable when a high tenacity material is used in the filament.